Fabrication techniques in the semiconductor industry currently utilize line widths below 1 micrometer (um) and layer thicknesses below 0.1 um. Due to these small dimensions, airborne contamination in the form of small particles can cause defects in semiconductor devices when the airborne contaminants come into contact with the semiconductor during the manufacturing process. Therefore, the environmental conditions under which semiconductor devices are manufactured are monitored closely to avoid such contamination. Typically, the semiconductor devices are manufactured in "clean room" type environments. To maintain the clean room status, the room is monitored to detect and control airborne contamination. Careful monitoring thereby aids the quality control of the semiconductor manufacturing process. As is well known in the art, there are multiple techniques for monitoring and sizing particles suspended in air, or more generally in a fluid. Optical detection of light scattered by the particles (hereafter referred to as "light scattering" for convenience) is often used.
Typically, a light scattering particle counter draws a sample of air through a beam of light. The particles in this sample flow of air scatter light in proportion to their size, shape and index of refraction. Refractive, reflective, or other light collection techniques are used to enhance the collection of light and focus it onto a photoelectric device. The photoelectric device converts the scattered light into an electrical signal. The created electrical signal is related to the amount of incident light and thus the particle size. Additionally, the signal is typically a pulse, wherein the signal width represents the velocity of the particle and the beam width. By accumulating the pulses over a period of time, the concentration of particles in the sample air flow may be determined.
The theoretical particle size limit for detecting particles by this method is approximately 0.05 um (particle diameter) (See e.g., Knollenberg and Luehr, 1976). However, those skilled in the art typically recognize the practical limit of this technique, at this time, to be 0.1 um.
One typical light scattering device is the Model 3755 LPC Laser Particle Counter manufactured by TSI, Inc., of St. Paul, Minn., the assignee of the present invention. This device utilizes a plurality of lenses to collect light scattered by contaminant particles in an air flow passing through a laser beam. The collected light is focused onto a single detector. The detector generates electrical signals in response to the incident light, with the signals being subsequently counted. Using this arrangement, the device is capable of measuring particles down to 0.5 um. However, as noted above, the trend toward smaller semiconductor fabrication techniques is continuing, therefore instruments capable of measuring smaller particles are required. There is also a trend to decrease the size of clean rooms. As the size of clean rooms' reduce to small work stations, the need for smaller, more compact and more reliable monitoring devices arises.
A further limitation of typical light scattering devices is the devices' sizing capabilities. This limitation is due to the devices' limited ability to collect the scattered light. As is well known, if a large percentage of the scattered light is detected, then a better indication of pulse intensity and thereby particle size is achieved. Optimally, all directional components (i.e., forward, backward and side) of the scattered light should be collected. By increasing the total collected light, the signal to noise ratio of a device may be improved.
Another drawback of many of these light scattering devices is that they use expensive optics to gather the scattered light. Also, these devices may indicate the presence of particles when none are present due to their limited finite electrical noise immunity, as well as by measuring ionizing radiation incident on the devices' detector(s).
Therefore, there arises a need for a light scattering device which is smaller and remedies the above described drawbacks.